Execute the jar compiled by Gradle, cause "there is no main class" - gradle

Question

The version of the gradle is 6.3-all. And here is my build.gradle:
plugins {
id 'java'
id 'org.jetbrains.kotlin.jvm' version '1.3.72'
}
group 'com.dennis'
version '1.0-SNAPSHOT'
repositories {
mavenLocal()
mavenCentral()
}
jar {
from {
configurations.runtime.collect { zipTree(it) }
}
manifest {
attributes 'Main-Class': 'com.dennis.tcp.robot.RobotClient'
}
}
dependencies {
/*...*/
}
compileKotlin {
kotlinOptions.jvmTarget = "1.8"
}
compileTestKotlin {
kotlinOptions.jvmTarget = "1.8"
}
tasks.withType(JavaCompile) {
options.encoding = "UTF-8"
}
And here is settings.gradle
rootProject.name = 'robot'
And here is the main class:
package com.dennis.tcp.robot;
/*import ignored*/
public class RobotClient {
private static final String URL = "192.168.10.140";
private static final int PORT = 9760;
public static void main(String[] args) {
NioEventLoopGroup group = new NioEventLoopGroup();
Bootstrap bootstrap = new Bootstrap();
try {
bootstrap.group(group)
.channel(NioSocketChannel.class)
.option(ChannelOption.SO_KEEPALIVE,true)
.handler(new RobotClientInitializer());
ChannelFuture future = bootstrap.connect(URL, PORT).sync();
future.channel().closeFuture().sync();
} catch (InterruptedException e) {
e.printStackTrace();
}finally {
group.shutdownGracefully();
}
}
}
The packaging progress went well but the output jar cannot be executed and give the Exception of no main class
The jar file shows the main class RobotClient is compiled indeed. However, the java -jar cannot execute or find the mainclass.
Why did this happend?